The present invention relates to in-mold pad printing, and more specifically to the pad printing of molded pieces while in the mold of an injection molding machine.
Prior to the present invention, fixing symbols, especially UPC symbols, on injection molded pieces has been accomplished by one of two methods. The first, which can be accomplished either manually or semiautomatically, is to adhere a sticker of the symbol directly onto the molded piece. This method has resulted in many stickers becoming detached from the molded pieces, which is undesirable, due to shaking and vibration during shipping.
The second method by which symbols have been placed on molded pieces is printing an ink image directly onto the molded piece using a printing apparatus that prints on the molded pieces after discharge from the injection molding machine. This method results in significant cost for the purchase, operation, and maintenance of the printing apparatus, as well as time and labor to transport the molded pieces from the injection molding machine to the printing apparatus.
Pad printing has been used for such things as pharmaceutical capsule parts, as in U.S. Pat. No. 3,910,183. In the method disclosed in that patent, cylindrical hard-shell capsule parts are formed on a row of pins of a capsule pin bar. The method uses a dip molding process and the printing does not take place within the normal cycle of the molding process. The capsules are instead transferred, either automatically or manually, to the printing station, thus adding time and expense to the procedure.
A particular type of pad printing has also been used to print UPC symbols, such as that disclosed in U.S. Pat. No. 4,473,008. The process uses an inking plate capable of altering its surface such that different images can be produced during each printing cycle. However, the actual printing or transfer of the inked image onto a specific product does not take place within the normal manufacturing parameters of a molding cycle. Furthermore, the process is limited as to how many products can be printed at a time.
Soviet document SU 290662 discusses a device and process for printing in a mold. The process uses a unit constructed in the form of a drum for applying ink, and which can rotate and move with respect to the mold into which there is fastened a set of printing elements which are sectors of the half-mold. Such a process is adequate for blow molding, but not injection molding. The printing elements used in SU 290662 are parts of the mold and not a separate ink transfer device. Instead of transferring an inked image after the product is hardened, the actual transfer of the inked image is performed as the product is being molded, which does not allow, before printing takes place, any products to be discarded that were not molded to specifications.
Accordingly, it is a desire to have a process where molded parts are printed while they are still in an injection molding machine so that time and cost can be saved while producing a superior printed product.